Press the information button (in the system icon bar) and change length from inch to mm.

Then go to Edit, choose Transform parents.
In the dropdown list choose scaling with 3 factors
Choose the coordinate system, then a point to scale to or choose origin on coordinate system.
Scaling factor along X: 1/25.4
Scaling factor along Y: 1/25.4
Scaling factor along Z: 1/25.4